Still, they have to get along on a far more fundamental level: They have to convince the reader that they all fit the setting. The omnium gatherium of D&D with both dragons and dinosaurs produces real difficulties. (I once read a D&D tie-in that convinced me that the author had used the tables in the first edition to roll up his monsters, they were that ill-suited.)
Avoiding the full fledged monsters -- the dragons, the manticores, the gryphons, etc. -- may make it easier. Using real animals, or even humans, as the basis and adding monstrous traits can do quite well. A wild boar the size of a peasant's cottage could make a quite good monster on its own.
